The use of nuclear quadrupole resonance (NQR) as a means of detecting explosives and other contraband has been recognized for some time. See, e.g., T. Hirshfield et al, J. Molec. Struct. 58, 63 (1980); A. N. Garroway et al, Proc. SPIE 2092, 318 (1993); and A. N. Garroway et al, IEEE Trans. on Geoscience and Remote Sensing 39, 1108 (2001). NQR provides some distinct advantages over other detection methods. NQR requires no external magnet such as required by nuclear magnetic resonance, and NQR is sensitive to the compounds of interest, i.e. there is a specificity of the NQR frequencies.
One technique for measuring NQR in a sample is to place the sample within a solenoid coil that surrounds the sample. The coil provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample and results in their producing their characteristic resonance signals. This is the typical apparatus configuration that might be used for scanning mail, baggage or luggage.
There is also a need, however, for a NQR detector that permits detection of NQR signals from a source outside the detector, e.g. a wand detector, that could be passed over persons or containers as is done with existing metal detectors, or a panel detector that persons could stand on or near. Problems associated with such detectors using conventional systems are the decrease in detectability with distance from the detector coil and the associated equipment needed to operate the system.
A detection system can have one or more coils that serve as both excitation and receive coils, or it can have separate coils that only excite and only receive. An excitation, i.e. transmit, coil of an NQR detection system provides a radio frequency (RF) magnetic field that excites the quadrupole nuclei in the sample; and results in the quadrupole nuclei producing their characteristic resonance signals that the receive coil detects.
It can be especially advantageous to use a receive coil, i.e. a sensor, made of a high temperature superconductor (“HTS”) rather than copper since the HTS self-resonant coil has a quality factor Q of the order of 103-106. The NQR signals have low intensity and short duration. In view of the low intensity NQR signal, it is important to have a signal-to-noise ratio (S/N) as large as possible. The signal-to-noise ratio is proportional to the square root of Q so that the use of a HTS self-resonant coil as a sensor results in an increase in S/N by a factor of 10-100 over that of a copper coil. Therefore, the use of a high temperature superconductor receive coil with a large Q provides a distinct advantage over the use of an ordinary conductor coil.
Separate excitation and receive coils having the same resonance frequencies result in a coupling between the coils. This coupling can result in interference with the performance of the coils as well as damage to the receive coils.
An object of this invention is thus to provide a method for reducing the coupling between the excitation and receive coils in a NQR resonance detection system.